Development of electrostatic images

ABSTRACT

An improvement in the developing means of an electrostatic copier apparatus using a liquid developer station having a pair of spaced upper and lower metallic electrodes between which the paper bearing a latent electrostatic image passes, such improvement comprising a smooth, non-metallic screen made of a low friction, low dielectric material, such as Nylon or Teflon, which screen is drawn tightly over the surface of the upper electrode which lies opposite the image bearing surface of the paper. The use of such a screen prevents any contact between the paper surface and the electrode surface and permits the distance between the upper and lower electrodes to be maintained at a relatively small value so that the image intensifying effect of the electrodes is maintained.

United States Patent MacKenzie [72] Inventor: Alfred K. MacKenzie, Wayland,

Mass. [73] Assignee: Dennison Manufacturing Company, Framingham, Mass. [22] Filed: Aug. 14, 1970 [21] Appl. No.: 63,884

[52] US. Cl. ..355/10, 118/637 [51] Int. Cl. ..G03g 15/10 [58] Field of Search ..355/10, 17; 118/637 [56] References Cited UNITED STATES PATENTS 3,249,088 5/1966 Ostensen ..118/637 3,146,145 8/1964 Kinsella ..l56/272 3,359,945 12/1967 Hastings et a1. ..118/637 2,952,241 9/1960 Clark et a1. ..118/637 [15] 3,682,542 [4 1 Aug. 8, 1972 Primary ExaminerfiSamuel S. Matthews Assistant Examiner-Richard L. Moses AttorneyRoberts, Cushman and Grover 57 ABSTRACT An improvement in the developing means of an electrostatic copier apparatus using a liquid developer station having a pair of spaced upper and lower metallic electrodes between which the paper bearing a latent electrostatic image passes, such improvement comprising a smooth, non-metallic screen made of a low friction, low dielectric material, such as Nylon or Teflon, which screen is drawn tightly over the surface of the upper electrode which lies opposite the image bearing surface of the paper. The use of such a screen prevents any contact between the paper surface and the electrode surface and permits the distance between the upper and lower electrodes to be maintained at a relatively small value so that the image intensifying effect of the electrodes is maintained.

8 Claims, 1 Drawing Figure DEVELOPMENT OF ELECTROSTATIC IMAGES This invention relates generally to photocopier apparatus of the type which utilizes electrostatically sensitized paper and a liquid developer and, more particularly, to an improved mechanism for developing the image bearing sheet as the sheet is conveyed through the developer station without damage to the image thereon.

Although not limited thereto, one form of an apparatus which can be appropriately adapted to use the system of the invention is illustrated by the type of apparatus generally shown in U.S. Pat. No. 3,425,777 previously issued on Feb. 4, 1969 to K]. White. As can be seen in that patent, an electrostatically sensitized paper is appropriately charged and exposed to a record image to form a latent electrostatic image thereon. The paper is then passed through a developer station along a path which dips into a tray of developing liquid with the latent image on the upperside of the sheet. 'In order to intensify the image at the developing station, upper and lower metallic members are provided in the developer pool above and below the electrostatically sensitized sheet material, such metallic members having faces opposed to the upper and lower surfaces of the paper, respectively, and extending in close juxtaposition to such surfaces throughout substantially the width of the paper. The two metallic members which are spaced as closely as possible together cooperate to produce an intensified image on the paper. Preferably the developer is supplied to the pool above the upper metallic member, the overall developer tray having drain openings in its bottom and the upper metallic member having openings therein so that the developer may flow therethrough to the upper side of the paper and thence through the drain openings. Such a structure is disclosed by U.S. Pat. No. 3,359,945 originally issued to D. B. Hastings et al, and now withdrawn from issue and filed as Reissue application Ser. No. 822,065 of A. K. MacKenzie.

A. problem which has arisen in utilizing such a structure is that the upper side of the sheet bearing the latent electrostatic image has sometimes come into contact with the upper metallic electrode as it passes through the slot, or gap, formed between the upper and lower electrodes. Such sliding contact of metal to image surface may cause the toner which is deposited on the latter surface either to be wiped ofi, thus destroying a portion of the image, or such contact may cause triboelectric charging and produce black lines on the image. if the electrodes are moved a greater distance away from each other so that the slot is widened in order to prevent contact between the upper electrode and the image bearing surface, the intensifying effect of the electrodes is reduced considerably and such increase in distance between electrodes is found to produce unsatisfactory results.

This invention provides a means for preventing contact between the image bearing surface and the oppositely disposed surface of the upper metallic electrode while still maintaining an effectively narrow gap or slot between the upper and lower electrodes in order to retain the image intensifying advantages thereof. In a preferred embodiment of the invention, the upper electrode surface is covered with a fine mesh, smooth, nonmetallic screening material which is drawn tightly over the working surface of the upper metallic electrode.

Contact of the image surface with such screening material does not produce the undesirable effects mentioned above and, thus, the use of such screening enables the electrodes to be placed extremely close together so as to obtain the improved intensifying action thereof.

The invention can be described in more detail with reference to the accompanying drawing wherein a cross-sectional view of a portion of the developing means utilizing the invention in an electrostatic copier apparatus is shown.

The particular embodiment of the invention illustrated in the drawing comprises a base 10, a tank 11 supported on the base, the tank containing developer 12 up to a level 13, and having a cover 14 provided with an opening in which a support 15 is mounted by means of brackets 16. In the support 15 is mounted a tray 17. In its bottom the tray has slots and disposed in the slots are guides 19, the slots and guides usually extending obliquely to the median line of the tray. The guides are mounted on the support 15 by brackets 20 and 21 which extend into recesses in the ends of the guides. The tray 17 is supported on the guides as shown at 22 and 23. Clearance between the guides and the edge of the slots permit developer to drain out of the tray. The upper edges of the guides are parallel to the bottom of the tray and both are concave as shown, the upper edges of the guides 19 determining the path of the exposed electrostatically sensitized paper material 24 through the tray.

Disposed above the aforesaid path in close parallelism therewith is a metal electrode 25 suitably supported on the ends of the tray by brackets 26 having flanges extending over the ends of the electrode and ears 27 overlapping the upper edges of the electrode. The upper electrode may be a perforated metal plate or, as shown, a metal screen material. As the paper material 24 approaches the tray 17 it is sprayed with developer through a nozzle 30 which may have a slot or a series of apertures extending across the width of the paper. The developer is supplied to the nozzle from tank 11 by a pump (not shown) through a conduit 28 and a tube 29. The outlet 31 from the tube to the nozzle, which may comprise a slot or series of apertures, is directed toward a wall 32 in the nozzle for the purpose of producing turbulence and thereby causing the stream of developer delivered by the nozzle to be distributed uniforrrrly across the paper. Developer is pumped into tray 17 faster than it can drain through the slots therein so as to maintain the tray full, the excess flowing over the ends of the tray.

The developer and tray 17 may be appropriately maintained at the level 13 in any manner known in the art as, for example, the manner described in my above mentioned Reissue application Ser. No. 822,065. For clarity such structure is not shown in detail herein. After leaving the pool of liquid in the tray 17, the developed paper material 24 passes between squeegee rolls 33 and 34 which may also serve to feed the material through the tray. Between the tray exit and the rolls is a V-shaped guide 35 which guides the material flatwise between the rolls so that the, material does not tend to wrinkle.

Thus, in passing through the developer station, the back of the material engages the guides 19 and 35 and the image side passes below upper metal electrode 25. In placing the two metallic members, that is, member 25 representing the upper electrode and the tray member 17, representing the lower electrode, as close as possible to the paper 24, such electrodes cooperate to produce intensification of the developed images without producing background cloudiness, the effect being greater than the sum of the effects of the metal electrode members if used alone.

In order to avoid any possibility of having the image or upper side of of the paper 24 contact the oppositely disposed surface of the upper metal electrode 25, a non-metallic screen member 36 is drawn tightly over the lower, or working, surface 37 of upper electrode 25 as shown. In the embodiment depicted, the edges 38 and 39 of the non-metallic screenare crimped between metal strips 40 and 41 which strips have suitable openings along the widths thereof into which are inserted elastic, or other spring-like, members 42 which draw the crimped edges together over the upper surface of electrode 25 so that the major portion of the screening is stretched tightly over the lower surface of such electrode.

The screen material is preferably made of a smooth, low friction, low dielectric, non-metallic substance, such as Nylon or Teflon. Although the mesh size of said screen may vary over a relatively wide range from No. 40 to No. 250, the screen is preferably made of a relatively fine mesh, that is, in a range from about No. 100 to about No. 250, a No. 157 mesh size being used successfully in a preferred embodiment of the invention.

It is also possible that alternative embodiments of the invention may be utilized. For example, the non-metallic screening may be deposited or screen printed in a mesh configuration or other network or parallel line design by utilizing a suitable non-conductive material which is so deposited or printed on the metallic electrode surface. If the metallic electrode is itself in the form of a screen-type electrode, an alternative embodiment may be utilized by coating the screen electrode wires with a suitable non-conductive coating material, such as Teflon, which would prevent contact with the metallic screen of the electrode but permit the metallic electrode action to occur.

What is claimed is:

1. For developing electrostatic images on a sheet of material, apparatus comprising a container for a pool of developer, means for guiding the sheet through the pool along a predetermined path which dips into the pool with the image side of the sheet facing upwardly, upper and lower metallic members in the pool juxtaposed to the upper and lower sides of the path, respectively, means for flowing developer onto the image side of said sheet, and non-metallic means formed in a mesh-like pattern and substantially covering the entire surface of said upper member oppositely disposed to said image side of said sheet.

2. Apparatus according to claim '1 wherein said non metallic means is in the form of a screen attached to said upper member and having a mesh size in a range from about No. 40 to about No. 250.

3. Apparatus according to claim 2 wherein said nonmetallic screen has a relatively fine mesh size in a range f botN.l00t bot ,250.

qi pp rat s acco1 di ng t o 8 131111 1 wherein said nonmetallic material is a low friction, low dielectric substance.

5. Apparatus according to claim 3 wherein said material is Nylon.

6. Apparatus according to claim 3 wherein said material is Teflon.

7. Apparatus according to claim 1 wherein said nonmetallic material is deposited on said oppositely disposed surface of said upper member in,a mesh-like pattern.

8. Apparatus according to claim 1 wherein said upper member is in the form of a wire screen and said non-metallic member is coated on the wires of said screen. 

1. For developing electrostatic images on a sheet of material, apparatus comprising a container for a pool of developer, means for guiding the sheet through the pool along a predetermined path which dips into the pool with the image side of the sheet facing upwardly, upper and lower metallic members in the pool juxtaposed to the upper and lower sides of the path, respectively, means for flowing developer onto the image side of said sheet, and nonmetallic means formed in a mesh-like pattern and substantially covering the entire surface of said upper member oppositely disposed to said image side of said sheet.
 2. Apparatus according to claim 1 wherein said non-metallic means is in the form of a screen attached to said upper member and having a mesh size in a range from about No. 40 to about No.
 250. 3. Apparatus according to claim 2 wherein said non-metallic screen has a relatively fine mesh size in a range from about No. 100 to about No.
 250. 4. Apparatus according to claim 1 wherein said non-metallic material is a low friction, low dielectric substance.
 5. Apparatus according to claim 3 wherein said material is Nylon.
 6. Apparatus according to claim 3 wherein said material is Teflon.
 7. Apparatus according to claim 1 wherein said non-metallic material is deposited on said oppositely disposed surface of said upper member in a mesh-like pattern.
 8. Apparatus according to claim 1 wherein said upper member is in the form of a wire screen and said non-metallic member is coated on the wires of said screen. 